Recent technological advances have led to weight saving, downsizing, and performance improvement of mobile terminals such as smart phones and tablets. These electronic devices use connectors as electric connecting members, and the connectors have gold plating films formed on the surfaces thereof. Gold is very excellent in physical (soft), chemical (very stable), and electric (low resistance) properties, and is therefore widely used not only for connectors but also for other electronic components such as printed circuit boards.
The plating processing of such a connector is performed by applying a nickel coating to a copper-based material and then applying a hard gold plating to the nickel coating. At present, such a hard gold plating is applied to a relatively large portion. However, in recent years, the price of gold has increased, and therefore there has been strong demand for reducing the amount of gold used in a gold plating process to reduce manufacturing costs. That is, there has been demand for establishment of a technique for forming a thin gold plating film only on a portion that needs to be plated. Various measures have been taken against plating apparatuses and gold plating solutions to achieve such a reduction in the amount of gold used.
Plating apparatuses have taken the approach that a gold plating solution is sprayed at a high speed from a minute nozzle only on a portion that needs to be plated with gold or the approach that a plating jig is used which is formed so that a gold plating solution comes into contact only with a portion that needs to be plated with gold.
Gold plating solutions have taken the approach that the concentration of gold in a gold plating solution is reduced to reduce the amount of gold lost by discharging the gold plating solution adhered to an object to be plated into a washing tank used in the next process. However, when the concentration of gold in a gold plating solution is reduced, the stability of a gold complex in a plating bath is reduced due to an increase in a bath voltage. This leads to a problem that gold particles are generated so that gold is deposited on the inner wall of a plating tank.
Patent Literatures 1 to 3 disclose gold plating solutions intended to reduce the amount of gold to be used. Patent Literatures 1 and 2 disclose gold plating solutions intended to inhibit the deposition of gold at a low current density to inhibit so-called plating leakage that is a phenomenon in which when a gold plating solution is sprayed on an object to be plated, a small amount of the gold plating solution comes into contact also with a portion that does not need to be plated with gold so that a plating film is formed on such a portion. Further, Patent Literature 3 discloses a gold plating solution that contains an organic additive so that a uniform gold plating film can be formed while the formation of pinholes is inhibited even when the gold plating film has a small film thickness.
The above-described inventions have considerably advanced the technique of reducing the amount of gold used during gold plating performed by applying an electric current to a gold plating solution. However, there is a case where gold is deposited on a nickel base coat by a displacement reaction even when an electric current is not applied to a gold plating solution before and after the gold plating process, which has become a serious problem in recent years. With an increase in the speed of gold plating, a gold plating solution is sprayed on an object to be plated at a high speed with the use of a pump. At this time, the gold plating solution leaks or splatters so that a mist of the gold plating solution adheres to a nickel-coated portion around a plating jig. This adhered gold plating solution forms a gold plating film on a nickel base coated portion that does not need to be plated with gold. Gold is a noble metal, and therefore has a lower ionization tendency than nickel as a base coat. Therefore, nickel is dissolved into the gold plating solution as nickel ions, and gold in the gold plating solution is deposited as a gold film on the nickel base coat. This deposition of gold caused by a displacement reaction is required to be improved from the viewpoint of quality and cost.
One of measures against this problem is to perform gold removal treatment using a gold remover on the entire surface of an object to be plated after the completion of gold plating. A gold plating film formed on a portion to be plated and a gold plating film formed on a portion that does not need to be plated are different in film thickness. Therefore, gold removal treatment is slightly performed on the entire surface of the object to be plated so that the gold plating film formed on a portion that does not need to be plated can be completely removed while the gold plating film formed on a portion to be plated is left so as to have a predetermined film thickness.
However, the thicknesses of gold films have been reduced in recent years, and therefore a difference in film thickness between a gold plating film formed on a portion to be plated and a gold plating film formed on a portion that does not need to be plated has become smaller. Therefore, in some cases, such gold removal treatment using a gold remover is not sufficiently effective.
Patent Literature 4 discloses an agent for inhibiting gold displacement/electric corrosion, and the agent contains a mercapto compound. The mercapto compound has the effect of inhibiting displacement in the initial state, but a decomposition product generated during running reduces the effect of inhibiting displacement.